1. Field of the Invention
The present invention generally relates to a negative pressure booster used for a toggle operation of a brake master cylinder of a motor vehicle. More particularly, the present invention relates to an improvement of a negative pressure booster equipped with an electromagnetic auxiliary control means, including a booster shell, a booster piston which is accommodated in the booster shell and partitions the inside thereof into a front negative pressure chamber communicating with a negative pressure source and a rear operation chamber, and a valve cylinder slidably supported on the rear wall of the booster shell and connected to the booster piston, wherein within the valve cylinder are arranged an input member movable to-and-fro, a control valve for communicating the operation chamber with the negative pressure chamber or air in accordance with the to-and-fro movement of the input member and an electromagnetic auxiliary control means for increasing the operation quantity of the control valve when it is excited while the input member advances.
2. Background Description
Laid open Japanese application JP-A-7-251733 discloses a negative booster equipped with an electromagnetic auxiliary control which operates the electromagnetic auxiliary control means at the time of urgent braking to increase the operation quantity of a control valve so that a large quantity of air is swiftly introduced into the operation chamber, thereby permitting an output rod to provide a high output to a toggle limit. The negative pressure booster disclosed in the above publication is provided with valve elements for introducing air into the operation chambers; i.e., a first valve element fixed to the input member and a second valve element coupled with the electromagnetic auxiliary control means. At the time of normal braking, as the input member advances, the second valve element opens after the first valve element opens completely.
In the negative pressure booster disclosed in the above publication, the quantity of advancement of the input member until the first valve element completely opens means an invalid stroke. In operation, a time delay occurs from when the input member starts to advance to when air is introduced into the operation chamber to operate a booster piston. Also, because of provision of two valve elements for air introduction, a large number of components are required, thus leading to complexity of the structure of the booster.